Technical Field
The present invention relates to polyacetal-based blends containing a polyacetal polymer, a thermoplastic polyurethane, and at least one thermoplastic crystalline polymer selected from the group consisting of nylon 612, nylon 6, polypropylene, and certain polyalkylene terephthalates containing certain ethylene copolymers. The blends are characterized as having improved toughness and improved elongation over that of a comparable blend of the polyacetal and the thermoplastic crystalline resin in the absence of the thermoplastic polyurethane.
Polyacetal compositions, which are also referred to in the art as polyoxymethylene compositions, are generally understood to include compositions based on homopolymers of formaldehyde or of cyclic oligomers of formaldehyde, for example, trioxane, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligorners of formaldehyde, with oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent.
Compositions based upon polyacetal of relatively high molecular weight, i.e., 20,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g., compression molding, injection molding, extrusion, blow molding, melt spinning, stamping, and thermoforming. Finished products made from such polyacetal compositions possess extremely desirable physical properties, including high stiffness, strength, and solvent resistance.
Crystalline thermoplastic resins also have many useful properties and enjoy wide commercial acceptance. With the increased concern about discarding plastic materials rather than recycling such materials, it has become more socially responsible to re-use plastic materials, or to use scrap plastic materials, rather than simply disposing of such materials. However, scrap plastic materials often contain more than one type of plastic material. Re-use of such scrap plastic materials normally requires that the individual components of the plastic scrap be segregated. This is an expensive step. As such, it has become desirable to find a way to merely mix together scrap plastic materials, without segregating the components of the scrap material, and form therefrom useful objects having good physical properties. It has long been recognized, however, by those skilled in the art of blending technology, that blends prepared from only two different types of resins often have such reduced physical properties that they are precluded from being useful. As such, partly due to the desire to find uses for scrap plastic materials, which often inherently contain more than one type of resin, efforts are being made to find polymers that are sufficiently compatible with each other that they can be blended together without suffering a significant losses in physical properties.
Polyacetal has been among the last of the crystalline engineering resins to be blended with other resins. Commercial blends of polyacetal and other resins, for purposes other than toughening, are relatively unknown. Generally, when polyacetal is blended with another resin, the physical properties of the polyacetal are significantly decreased. However, by the present invention, it has been found that polyacetal containing certain thermoplastic crystalline resins, such as would be present in scrap plastic streams, can be blended with thermoplastic polyurethanes and the resultant blend has better toughness and elongation properties than does the scrap polyacetal containing the crystalline resin alone. More significantly, this implies that a means has been found to allow for the recycling of plastic waste streams that contain a major portion of polyacetal and minor portions of certain other common crystalline resins by blending said waste with a thermoplastic polyurethane and forming therefrom objects having useful levels of toughness. Such objects that can be prepared from the blends of the present invention include agricultural and construction objects, such as benches and pallets.